The planographic printing plate material for CTP, which is inexpensive, can be easily handled, and has a printing ability comparable with that of a PS plate, is required accompanied with the digitization of printing data. Recently, a processless planographic printing plate material which can be applied to a printing press employing a direct imaging (DI) process without development by a special developing agent has been required.
As a method of forming an image in the processless planographic printing plate material, a useful one forms an image employing an infrared laser. The methods are divided into three kinds detailed later, i.e., a method employing a non-processing type planographic printing plate material, a method employing an ablation type planographic printing plate material, and a method employing a development on-press type heat fusible image formation planographic printing plate material.
Examples of the non-processing type planographic printing plate material include a planographic printing plate material disclosed in JP 3064807 and 3206297, which comprises a cross-linked hydrophilic resin layer capable of being made lipophilic by heating. This material is ideal as a processless planographic printing plate material, but is difficult to make a difference between hydrophilicity and lipophilicity in the resin layer after image formation, and has problems in that ink receptivity is insufficient at image portions, and stain is likely to occur at non-image portions.
Examples of the ablation type planographic printing plate material include those disclosed in for example, Japanese Patent O.P.I. Publication Nos. 8-507727, 6-186750, 6-199064, 7-314934, 10-58636 and 10-244773. These references disclose a planographic printing plate material comprising a substrate and a hydrophilic layer or a lipophilic layer as an outermost layer. In the planographic printing plate material having a hydrophilic layer as an outermost layer, the hydrophilic layer is imagewise exposed to imagewise ablate the hydrophilic layer, whereby the lipophilic layer is exposed to form image portions. However, there is problem that contamination of the interior of the exposing apparatus by scattered matter caused by the ablation of the surface layer. In order to prevent such a scattered matter caused by the ablation of the surface layer, there is a planographic printing plate material in which a water soluble protective layer is further provided on the hydrophilic layer wherein the ablated layer is removed together with the protective layer on the printing press. The ablation type planographic printing plate material, in which it is possible to design a hydrophilic layer and an oleophilic layer, individually, produces no serious problem regarding ink receptivity at image portions and stain at non-image portions, however, has problem in that it is low in sensitivity, forms an image with low resolution and low quality, and may produce fringes at image portions.
As a method developing a development on-press type heat fusible image formation planographic printing plate material, the following method is known: a method (see JP 2938397) of imagewise exposing a planographic printing plate material comprising a hydrophilic substrate and provided thereon, an image formation layer containing a hydrophobic thermoplastic polymer particles dispersed in a hydrophilic binder, which is heat fusible, or a planographic printing plate material comprising a hydrophilic substrate and provided thereon, an image formation layer or its adjacent layer, containing a light-to-heat conversion material; mounting the imagewise exposed planographic printing plate material on a plate cylinder of a printing press; and then developing the resulting planographic printing plate material by providing dampening water and/or printing ink on the material while rotating the plate cylinder to manufacture a planographic printing plate, or a method (see JP 2938398) of mounting, on a plate cylinder of a printing press, a planographic printing plate material comprising a hydrophilic substrate and provided thereon, an image formation layer containing a hydrophobic thermoplastic polymer particles dispersed in a hydrophilic binder, which is heat fusible, or a planographic printing plate material comprising a hydrophilic substrate and provided thereon, an image formation layer or its adjacent layer, containing a light-to-heat conversion material; imagewise exposing the planographic printing plate material, employing laser or LED; and then developing the resulting planographic printing plate material by providing dampening water and/or printing ink on the material while rotating the plate cylinder to manufacture a planographic printing plate. These methods employ a planographic printing plate material comprising a hydrophilic layer or a grained aluminum plate and provided thereon, an image formation layer containing thermoplastic particles and a water soluble binder, which is higher in sensitivity, dissolving power, and image quality than a planographic printing plate material of ablation type.
As a planographic printing plate material enhancing printing durability, there is a planographic printing plate material comprising a hydrophilic layer surface, and provided thereon, an image formation layer containing a hydrophobic thermoplastic particles, a hydrophilic polymer binder, (which is water soluble, water dispersible, alkali soluble or alkali dispersible), and optionally an infrared absorbing compound, wherein the hydrophobic polymer particles comprise a material having one selected from the group consisting of an amido group, a urethane group, a methacrylonitrile residue, a chrotononitrile residue, a vinylidene cyanide residue, an isocytosine residue, a pyrrolidone residue, a piperazine fesidue, a cyanomethyl group, a cyanoethyl group, a cyanopropyl group, and a cyanoaryl group (see Japanese Patent O.P.I. Publication No. 2002-251005).
At present, a processless planographic printing plate material, which is considered to have a sufficient printability, is subjected to development on a printing press (development on-press) to form an image. The development on-press is a process providing dampening water and/or printing ink on an exposed planographic printing plate material mounted on a plate cylinder of a printing press bringing an ink roller or a dampening water roller into contact with the material while rotating the plate cylinder to remove a part of the image formation layer. Further, a part of the image formation layer may be removed by bringing the plate cylinder into contact with a blanket of a printing press. In this case, as the image formation layer incorporated into printing ink is gradually transferred onto a printing paper sheet through the blanket, a serious problem is not caused as long as the image formation layer is not extremely colored.
In contrast, as the image formation layer incorporated into dampening water is accumulated in the dampening water, problem may be produced that printing conditions vary, when printing is continued for a long term. As dampening water is usually contaminated with ink emulsion or paper powder during printing, dampening water filtered and re-circulated during printing is employed. There is description in JP 2938397 and JP 2938398 above that an image formation layer to be subjected to development on-press contains thermoplastic particles with a particle size of 90 nm, and there is description in Japanese Patent O.P.I. Publication No. 2002-251005 above that an image formation layer to be subjected to development on-press contains thermoplastic particles with a particle size of 65 nm. However, in a conventional filtration of dampening water, incorporation of such minute particles into dampening water is not considered.
Accordingly, when printing has been carried out for a long term employing a printing press using a conventional filtration of dampening water and employing a processless planographic printing plate material, particularly a processless planographic printing plate material comprising an image formation layer containing hydrophobic precursor particles, composition of the dampening water varies, which may lead to change of printing conditions.
A dampening water cleaning device, purifying dampening water without requiring exchange of a filter medium clogged with floating matter, and preventing ink from accumulating in the dampening water, is known in which dampening water, overflowing a dampening water boat, is introduced to an intermediate tank, and then to a dampening water cooling and re-circulating tank, and re-supplied to the dampening water boat, wherein an ultrafiltration ceramic membrane of a cross-flow system is used as the filter medium (see Japanese Patent O.P.I. Publication No. 2002-59531). A technique is known in which removes fine particles of ink, oil or paper powder contained in dampening water through a potential adsorption filter and further bleaches and deodorizes the dampening water through an activated carbon filter to regenerate a dampening water, and reuses the regenerated dampening water, wherein the dampening water is recycled without being discarded (see Japanese Patent O.P.I. Publication No. 2000-351193). However, these techniques are not applied to a printing method employing a planographic printing plate material capable of being developed on a printing press.